Problem 19 - Chemistry: The Central Science

Question

Balance the following equations, and indicate whether they are combination, decomposition, or combustion reactions: (a) \(\mathrm{Al}(s)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{AlCl}_{3}(s)\) (b) \(\mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\) (c) \(\mathrm{Li}(s)+\mathrm{N}_{2}(g) \longrightarrow \mathrm{Li}_{3} \mathrm{~N}(s)\) (d) \(\mathrm{PbCO}_{3}(s) \longrightarrow \mathrm{PbO}(s)+\mathrm{CO}_{2}(g)\) (e) \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}_{2}(l)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\)

Step-by-Step Solution

Answer

(a) 2Al(s) + 3Cl₂(g) → 2AlCl₃(s) - Combination reaction (b) C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(g) - Combustion reaction (c) 6Li(s) + N₂(g) → 2Li₃N(s) - Combination reaction (d) PbCO₃(s) → PbO(s) + CO₂(g) - Decomposition reaction (e) C₇H₈O₂(l) + 9O₂(g) → 7CO₂(g) + 4H₂O(g) - Combustion reaction

Step 1: Identify reactants and products

Reactants: Al (s) and Cl₂ (g); Product: AlCl₃ (s)

Step 2: Balance the equation

To balance the equation, follow these steps: 1. Compare the number of moles of each element on the reactants and products side. 2. Adjust the coefficients, if necessary, to balance the elements. The balanced equation is: 2Al(s) + 3Cl₂(g) → 2AlCl₃(s)

Step 3: Determine reaction type

As two elements combine to form a single compound, this is a combination reaction. (b)

Step 4: Identify reactants and products

Reactants: C₂H₄ (g) and O₂ (g); Products: CO₂ (g) and H₂O (g)

Step 5: Balance the equation

To balance the equation, follow these steps: 1. Compare the number of moles of each element on the reactants and products side. 2. Adjust the coefficients, if necessary, to balance the elements. The balanced equation is: C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(g)

Step 6: Determine reaction type

As the reaction involves oxygen as a reactant and produce carbon dioxide and water, this is a combustion reaction. (c)

Step 7: Identify reactants and products

Reactants: Li (s) and N₂ (g); Product: Li₃N (s)

Step 8: Balance the equation

To balance the equation, follow these steps: 1. Compare the number of moles of each element on the reactants and products side. 2. Adjust the coefficients, if necessary, to balance the elements. The balanced equation is: 6Li(s) + N₂(g) → 2Li₃N(s)

Step 9: Determine reaction type

As two elements combine to form a single compound, this is a combination reaction. (d)

Step 10: Identify reactants and products

Reactant: PbCO₃ (s); Products: PbO (s) and CO₂ (g)

Step 11: Balance the equation

The equation is already balanced: PbCO₃(s) → PbO(s) + CO₂(g)

Step 12: Determine reaction type

As a single compound breaks down into two products, this is a decomposition reaction. (e)

Step 13: Identify reactants and products

Reactants: C₇H₈O₂ (l) and O₂ (g); Products: CO₂ (g) and H₂O (g)

Step 14: Balance the equation

To balance the equation, follow these steps: 1. Compare the number of moles of each element on the reactants and products side. 2. Adjust the coefficients, if necessary, to balance the elements. The balanced equation is: C₇H₈O₂(l) + 9O₂(g) → 7CO₂(g) + 4H₂O(g)

Step 15: Determine reaction type

As the reaction involves oxygen as a reactant and produce carbon dioxide and water, this is a combustion reaction.

Key Concepts

Equation Balancing

Chemical reactions need to be balanced to adhere to the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. This means the quantity of each element must be the same on both sides of the equation.

When balancing an equation, you must adjust the coefficients, which are the numbers placed before compounds, to make sure that the number of atoms for each element is equal on both sides of the equation.

Here is a simple way to balance a chemical equation:

• Identify the reactants and products in the equation.
• Count the number of atoms for each element on both the reactant and product sides.
• If the numbers do not match, adjust the coefficients to balance the atoms.
• Check to ensure the equation is balanced numerically and that the coefficients are in the lowest possible terms.

This process can be practiced using equations from the exercise, like \(2 ext{Al}(s) + 3 ext{Cl}_2(g) \rightarrow 2 ext{AlCl}_3(s)\).

Reaction Types

Understanding reaction types helps in identifying how substances interact in a chemical process. There are several fundamental types of chemical reactions:

• Combination Reaction: Two or more substances combine to form a single compound.
• Decomposition Reaction: A compound breaks down into two or more simpler substances.
• Combustion Reaction: A substance reacts with oxygen, often producing carbon dioxide and water.
• Single and Double Replacement Reactions: Elements or ions are exchanged between reactants.

By classifying reactions, it becomes easier to predict the products formed and to balance the chemical equations accurately. Identifying whether a reaction involves multiple reactants forming one product or one reactant breaking into multiple products can simplify this task.

Combination Reaction

A combination reaction occurs when two or more substances join together to form a new single product. This type of reaction is sometimes called a synthesis reaction.

Key features of a combination reaction include:

• They generally release energy in the form of heat or light.
• Result in the formation of a more complex molecule from simpler substances.

For example, when aluminum combines with chlorine, \(2 ext{Al}(s) + 3 ext{Cl}_2(g) \rightarrow 2 ext{AlCl}_3(s)\), it forms aluminum chloride, a single product. This specific reaction is useful in demonstrating how simple elements can combine to create a compound.

Decomposition Reaction

In a decomposition reaction, a single compound breaks down into two or more simpler substances. This type of reaction is typically endothermic, meaning it requires energy to proceed.

Important aspects of decomposition reactions are:

• These reactions often require heat, light, or electricity to break the bonds of the compound.
• The initial compound is usually more complex, and the products are simpler elements or compounds.

An example from the exercise is: \( ext{PbCO}_3(s) \rightarrow ext{PbO}(s) + ext{CO}_2(g)\). Here, lead carbonate decomposes into lead oxide and carbon dioxide, showcasing the breakdown of compound into simpler parts.

Combustion Reaction

Combustion reactions are characterized by a substance reacting with oxygen to produce energy in forms such as heat or light. This type of reaction typically involves hydrocarbons and results in carbon dioxide and water being products.

Main points to note about combustion reactions include:

• They are a major source of energy and have many industrial applications.
• They involve the element oxygen as one of the primary reactants.

A common example from the exercise is: \( ext{C}_7 ext{H}_8 ext{O}_2(l) + 9 ext{O}_2(g) \rightarrow 7 ext{CO}_2(g) + 4 ext{H}_2 ext{O}(g)\). This is a complete combustion reaction where a hydrocarbon burns in oxygen to yield carbon-based products and water.